[3.12] gh-112356: LOAD_GLOBAL can only include one PUSH_NULL (#112566)

Author: sweeneyde

Lib/test/test_peepholer.py

@@ -1075,5 +1075,26 @@ def test_no_unsafe_static_swap(self):
         ]
         self.cfg_optimization_test(insts, insts, consts=list(range(3)), nlocals=1)
 
+    def test_only_one_push_null_per_load_global(self):
+        # When optimizing func()(), a second pass shouldn't
+        # let the LOAD_GLOBAL absorb another PUSH_NULL.
+        before = [
+            ('PUSH_NULL', 0, 1),
+            ('PUSH_NULL', 0, 1),
+            ('LOAD_GLOBAL', 0, 1),
+            ('CALL', 0, 1),
+            ('CALL', 0, 1),
+            ('RETURN_VALUE', 0, 1),
+        ]
+        after = [
+            ('PUSH_NULL', 0, 1),
+            ('LOAD_GLOBAL', 1, 1),
+            ('CALL', 0, 1),
+            ('CALL', 0, 1),
+            ('RETURN_VALUE', 0, 1),
+        ]
+        self.cfg_optimization_test(before, expected_insts=after, consts=[])
+        self.cfg_optimization_test(after, expected_insts=after, consts=[])
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2023-11-30-23-47-56.gh-issue-112356.g2BqyK.rst

@@ -0,0 +1 @@
+Stopped erroneously deleting a ``LOAD_NULL`` bytecode instruction when optimized twice.

Python/flowgraph.c

@@ -1554,9 +1554,9 @@ optimize_basic_block(PyObject *const_cache, basicblock *bb, PyObject *consts)
             case KW_NAMES:
                 break;
             case PUSH_NULL:
-                if (nextop == LOAD_GLOBAL && (inst[1].i_opcode & 1) == 0) {
+                if (nextop == LOAD_GLOBAL && (bb->b_instr[i+1].i_oparg & 1) == 0) {
                     INSTR_SET_OP0(inst, NOP);
-                    inst[1].i_oparg |= 1;
+                    bb->b_instr[i+1].i_oparg |= 1;
                 }
                 break;
             default: